1. Field of the Invention.
This invention relates to printing machines in general, and more specifically to print head assemblies for impact matrix, or dot, printers.
2. Description of the Prior Art
In impact matrix printing apparatus, individual needle-like printing elements are thrown longitudinally to impact endwise against a recording medium, thereby forming a dot on the medium. Each such dot is usually part of a dot pattern which forms a given alphanumeric character, the pattern being selected from a matrix of possible dot positions. Typically, individual electromagnetic actuators are used for each printing needle, or wire, as they are often referred to. The actuators have to move the wire with sufficient force to impact the medium, and then withdraw the wire so it is clear of the medium. While speed is a major consideration in such printing apparatus, the impact force of the wire against the medium is also very important, since the force must be sufficient to produce the desired image. Clearly, a force sufficient to merely produce an image on a single sheet of paper when the wire is separated from the paper by only an ink-carrying ribbon is probably insufficient to produce the required image on each sheet when the medium is a plurality of sheets of paper with intervening sheets of carbon paper.
Prior impact needle actuators such as that shown in U.S. Pat. No. 3,592,311 to A. S. Chou et al. have used a printing wire associated with an electromagnet armature and a leaf spring. The wire is connected to the armature and the armature is connected to the leaf spring. An electromagnet holds the armature so that the leaf spring is in a flexed position. When the magnetic coil is deenergized the armature and the wire are propelled forward by the leaf spring to a printing position. Energizing the magnetic coil then retracts the wire from the printing position to a retracted position and at the same time flexes the leaf spring once again, recocking the actuator. Much the same principle is used in U.S. Pat. No. 3,672,482 issued to Brumbaugh et al. However, instead of having an electromagnet which is deenergized to release the armature and the spring, there is a permanent magnet whose field is overcome by activating an electromagnet having a field which is the reverse of the permanent magnet field and approximately equal in magnitude. Typically, one can expect a force of about one and one-half pounds from such spring-actuated printing wires. A typical operating speed is about 40 characters per second.
A somewhat greater force, for example about three pounds, can be obtained by a system which uses an electromagnet to impell the wire from a retracted position into a printing position. Typical operating speed for such an arrangement is about 165 characters per second. Normally, the wire is retained in a retracted non-printing position by a spring. The force created by the electromagnet overcomes the retaining force of the spring. For example, U.S. Pat. No. 3,584,575 issued to J. Distl teaches a wire connected to an armature which in turn is connected to a coil spring that holds it in a retracted nonprinting position. The armature is within a magnetic coil and near a core piece. When the magnetic coil is energized the coil and the core piece act to attract the armature and move it and the connected printing wire into a printing position. In the printing position, the spring is resiliently flexed from its normal position. De-energizing the magnetic coils allows the spring to return to its normal unextended position, which returns the print wire to a retracted, non-printing position. Similarly, U.S. Pat. No. 3,690,431 issued to R. Howard teaches a system where energization of the solenoid coil rapidly moves the printing wire in the impact printing direction, against the bias of a spring. In this particular patent the spring, instead of being a coil spring as in the Distl patent, is in the shape of a wagon wheel. The armature is connected to the hub of the wagon wheel, movement of the armature causing the spokes of which to elastically deflect relative to the rim.